JUPITER: Powerful impulse DPSS laser

Application

The laser is built for materials sputtering in vacuum in the course of nanostructure investigations at Ioffe Physico-technical Institute, RAS. The laser may also be used for speedy "on the flight" marking of fast moving objects (for instance, wire and cable insulation), for harmonics generation, for lidar systems, etc.

Pockels cell (E-O switch) is made on DKDP crystal base with thermostating of the crystal. The E-O gate is normally closed with high voltage 3-4.5 kV and in the moment of reaching max inversion the voltage drops for 10 ns, opening the gate. This results in short (20-25 ns) laser pulse of ~ 15 MW pulse power.

The Generator cavity is flat. But at the rate 100 Hz, it's being transformed into flat-spherical due to thermo-lensing effect causing the appearance in the rod the lense with focal length 1.4 m (at 90A pump current). The Generator laser module is located near the output mirror for increasing the output beam diameter. Intra-cavity diaphragm is absent, for due to good rod pump excitation map (it can be seen on the Photo 2, where luminescence from the top end of the rod presented ) near TEM00 mode (M2~1.5) is naturally generating. One pass weak signal generator rod amplification (at 90A) is more than 10.

Every of two Ø5 × 94 mm rods of Amplifier are being pumped from 5 sides with 25 Laser Diode bars with total power 2500W each.

The pump rod distribution for the laser modules of Amplifier is presented on Fig 3. Thermo lens - reprate frequency dependence at 90A is presented on Fig. 4.

On Fig. 5 one can see the scheme for measuring dependence of weak signal amplification - pump current in the cascade of Amplifier, and on the Fi. 6 - obtained results.

For Generator (MO) and Amplifier (PA) decoupling, as well as for protection of MO optical elements from depolarized part of PA radiation we used Faraday Cell optical insulator 9 based on permanent magnets and 6 mm aperture TGG crystal. 45 deg. quartz plate in front of it fits the azimuths of polarization of MO and FC.

1/4 wavelength phase 0-order quartz plate 13 rotates polarization plane of incident beam to orthogonal and as a result the laser radiation goes out through polarizer 4.

Note that, at high reprate (100 Hz) losses via depolarization of the beam in PA caused by thermo induced birefringence became noticeable. These losses may reach 30%, and near field on the laser outputе has the look «of Maltese cross« (Fig.6).

Fig.6. Near field radiation distribution, disturbed by losses on output polarizer due to induced birefringence, which appears in active elements of PA.

The losses were practically eliminated by placing between the PA laser modules the quartz plate rotating the polarization plane on 90 deg.

Fig 7. Near field (left) and far field (right) of the laser radiation in the focus of 100 cm lens.